1. Technical Field
The present application relates to an image processing method, an image processing apparatus, an image forming apparatus, an image reading apparatus, and a recording medium which allow an improvement in the result of ACS determination.
2. Description of the Related Art
As image forming apparatuses such as electronic copiers, not only a conventional analog type, but also a digital type have been prevalent. With the advancement of a digital image processing technology, a full-color digital copier which reproduces a high-quality color image has been available as a commercial product. In general, a full-color digital copier performs color reproduction using a plurality of toners, and also has a monochrome mode using only a black toner. Copying in the monochrome mode uses only the black toner, and is therefore lower in cost and shorter in copying time than color copying which superimposes a plurality of toner images in succession.
In consideration of this, an ACS (Auto Color Selection) function has been actually used which determines whether a document is a color document or a monochrome document, and sets a copy mode in accordance with the document (see, e.g., Japanese Patent Laid-Open No. 8-251402). This function is particularly useful when a large number of documents are copied using an ADF (Automatic Document Feeder). A user need not discriminate a color document from a monochrome document and, by merely setting the plurality of documents to the ADF and pushing a start key, color copying is performed for a color document, while monochrome copying is performed for a monochrome document.
For example, in an image processing apparatus described in Japanese Patent Laid-Open No. 8-251402, a lightness histogram of a document and a histogram of achromatic dots in the document are generated, and a chromatic histogram of the document is calculated from the difference between these histograms. Next, it is determined whether the document is a color document or a black-and-white document according to the ratio between the number of chromatic pixels and the number of achromatic pixels. The histograms are analyzed based on the result of the determination so that the document is classified into any of five types of documents which are a black-and-white picture document, a black-and-white standard document, a full-color picture document, a color standard document with a white page background, and a color standard document with a color page background. Then, a proper page background correction, a black text discrimination process, a gray scale reproduction process, and the like are performed in accordance with the document type.
However, the discrimination between the chromatic pixels and the achromatic pixels is made irrespective of the classification of each pixel (to which one of a plurality of components including a text component and a halftone component the pixel belongs), and the determination of the color document or the monochrome document is made using the result of the discrimination. Accordingly, the accuracy of discrimination is low, and therefore the quality of an image is low. For example, there are cases where a document containing a color text with low chroma is determined to be a monochrome image document, and where a document composed only of composite gray is determined to be a color image document.
As a method for solving such a problem, the applicant of the present application has proposed a method which changes a criterion for ACS determination based on the result of discriminating pixels in Japanese Patent Application No. 2007-120055.
However, in this case, when noise is present in the vicinity of a black text or there are density variations, the black text is determined to be a color text so that a monochrome (black-and-white) document is erroneously determined to be a color document. In the case with a picture (halftone picture) printed in a spot color (custom color) such as brown, the problem occurs that an intrinsically color document is erroneously determined to be a monochrome document.